The present invention relates to electronic delay lines, and more particularly to such delay lines that can be controlled to provide a controllable delay.
Electronic delay lines are used in many devices to delay the transmission of an electric signal. To achieve changes in the delay, some delay lines add or subtract delay elements to achieve different delay times, or adjust the corresponding delay elements in a delay line chain to obtain the desired delay time. The element tolerances need to be calibrated, and the choice is limited. One needs prior knowledge of the system to choose the elements necessary for proper delay time. Some programmable delay lines use analog-to-digital and digital-to-analog converter circuits to digitally control the delay time. The structure is rather complicated. In addition, the speed for digital conversion is slow. Also most importantly, such digital circuits typically cannot operate at microwave frequencies.
There are many applications for tunable delay lines. An example, of an application for such tunable delay lines is the feed-forward amplifier. Because of their superior linearity, feed-forward amplifiers are widely used in telecommunications. The theory for achieving such linearity is described as follows. A two-tone signal is fed into a power splitter. One output path from the power splitter is connected to an amplifier and the other output path is connected to a delay line. The output of the amplifier will have a certain delay time, signal gain, intermodulation products, and a 180-degree phase shift. The output of the delay line is still a linear signal without phase shift or intermodulation products. By setting the same delay time for both paths, and using a hybrid coupler to couple the output of the amplifier to the output of the delay line with the same amplitude, the two-tone signal will be cancelled by the phase difference but the intermodulation products will not be cancelled. The intermodulation products will then be amplified by a second amplifier to obtain a 180 degree phase sift. Meanwhile, part of the output from the first amplifier is fed to a coupler that connects to a second delay line. The delay time of the second delay line is made equal to the delay time of the second amplifier. Finally, the output of the second amplifier is coupled to the output of the second delay line with the same amplitude of the intermodulation products. The result is that the intermodulation products are cancelled but not the two-tone signal. Therefore, a linear signal is obtained. In this type of application, the delay time needs to be accurate, reliable, and easily controlled.
Previous patents relating to tunable/adjustable delay lines include U.S Pat. Nos. 4,701,714; 4,766,559; and 5,631,593. Programmable delay lines are shown in U.S. Pats Nos. 5,933,039; 5,923,197; 5,641,954; 5,900,762; 5,465,076; 5,355,038; 5,144,173; 5,140,688; 5,013,944; and 4,197,506.
Tunable ferroelectric materials are materials whose permittivity (more commonly called dielectric constant) can be varied by varying the strength of an electric field to which the materials are subjected. Even though these materials work in their paraelectric phase above the Curie temperature, they are conveniently called xe2x80x9cferroelectricxe2x80x9d because they exhibit spontaneous polarization at temperatures below the Curie temperature. Tunable ferroelectric materials including barium-strontium titanate (BST) or BST composites have been the subject of several patents.
Dielectric materials including barium strontium titanate are disclosed in U.S. Pat. No. 5,312,790 to Sengupta, et al. entitled xe2x80x9cCeramic Ferroelectric Materialxe2x80x9d; U.S. Pat. No. 5,427,988 to Sengupta, et al. entitled xe2x80x9cCeramic Ferroelectric Composite Material-BSTOxe2x80x94MgOxe2x80x9d; U.S. Pat. No. 5,486,491 to Sengupta, et al. entitled xe2x80x9cCeramic Ferroelectric Composite Material-BSTOxe2x80x94ZrO2xe2x80x9d; U.S. Pat. No. 5,635,434 to Sengupta, et al. entitled xe2x80x9cCeramic Ferroelectric Composite Material-BSTO-Magnesium Based Compoundxe2x80x9d; U.S. Pat. No. 5,830,591 to Sengupta, et al. entitled xe2x80x9cMultilayered Ferroelectric Composite Waveguidesxe2x80x9d; U.S. Pat. No. 5,846,893 to Sengupta, et al. entitled xe2x80x9cThin Film Ferroelectric Composites and Method of Makingxe2x80x9d; U.S. Pat. No. 5,766,697 to Sengupta, et al. entitled xe2x80x9cMethod of Making Thin Film Compositesxe2x80x9d; U.S. Pat. No. 5,693,429 to Sengupta, et al. entitled xe2x80x9cElectronically Graded Multilayer Ferroelectric Compositesxe2x80x9d; and U.S. Pat. No. 5,635,433 to Sengupta, entitled xe2x80x9cCeramic Ferroelectric Composite Material-BSTOxe2x80x94ZnOxe2x80x9d. These patents are hereby incorporated by reference. A copending, commonly assigned United States patent application titled xe2x80x9cElectronically Tunable Ceramic Materials Including Tunable Dielectric And Metal Silicate Phasesxe2x80x9d, by Sengupta, filed Jun. 15, 2000, discloses additional tunable dielectric materials and is also incorporated by reference. The materials shown in these patents, especially BSTOxe2x80x94MgO composites, show low dielectric loss and high tunability. Tunability is defined as the fractional change in the dielectric constant with applied voltage.
Many prior art tunable delay lines have complicated tuning structures or too many tuning elements, and the tolerance of each delay element may affect repeatability and stability. There is a need for tunable delay lines that are relatively simple in structure and can be rapidly controlled over a broad frequency range of operation.
Tunable delay lines constructed in accordance with this invention include an input, an output, a first conductor electrically coupled to the input and the output, a ground conductor, and a voltage tunable dielectric layer positioned between the first conductor and the ground conductor. DC blocks and impedance matching sections are connected between the first conductor and the input and output. Additional layers of tunable dielectric material and additional conductors can be positioned in parallel with the voltage tunable layer.